The present invention relates to a ceramic-sprayed member made of an aluminum alloy as a parent material and formed with a ceramic-sprayed layer on its surface and, more particularly, to a ceramic-sprayed member to be used as either a member or the piston head of an automotive engine which, for example, is required to have heat resistance and heat insulating properties or wear resistance.
In recent years, ceramic members, which have their heat resistances or heat insulating properties improved by spraying the surface of a parent material or aluminum alloy with a ceramic having a low thermal conductivity and excellent heat insulating properties to form a ceramic-sprayed layer, are used for a member to be partially heated to a high temperature, such as the piston of an automotive engine. The cylinder bore of the automotive engine or a member having a sliding surface required to have wear resistance is also formed with the ceramic-sprayed layer so as to improve the wear resistance of the surface of the parent material of an aluminum alloy.
In the member having the ceramic-sprayed layer on the parent material of the aluminum alloy, however, there is a substantial difference between the coefficients of thermal expansion of the aluminum alloy and the material of the ceramic-sprayed layer. This difference causes an engine member repeatedly heated and cooled to be repeatedly subjected to a high thermal stress. As a result, the interface between the ceramic-sprayed layer and the parent material and the inside of the ceramic-sprayed layer will crack until the ceramic-sprayed layer will possibly peel from the surface of the parent material. In case the ceramic-sprayed layer is formed on the sliding surface also required to have wear resistance, a shearing stress acts between the ceramic-sprayed layer and the parent material so that their interface will frequently crack until the ceramic-sprayed layer peels from the parent material.
In order to prevent these problems, therefore, there is a conventional process for forming the ceramic-sprayed layer on the parent material of an aluminum alloy. With a view to preventing the ceramic-sprayed layer from peeling, according to the known process (e.g., "Cummins/TACOM Advanced Adiabatic Engine", R. Kamo et al., SAE Papers No. 840428), the parent material is thinly sprayed in advance on its surface with an alloy, which has a coefficient of thermal expansion intermediate the parent or aluminum alloy and a ceramic and has excellent adherence to the ceramic, e.g., mainly a nickel-based alloy such as an alloy of Ni--Cr, Ni--Al, Ni--Cr--Al, Ni--Cr--Al--Y or Ni--Co--Cr--Al--Y to form a primary sprayed layer called the "bond layer" or "intermediate layer", and this primary sprayed layer is has its surface sprayed with the ceramic. Even when such primary sprayed layer is sandwiched between the parent aluminum alloy and the ceramic-sprayed layer, however, the adherency between the two is not sufficient. As a result, the parent material and the primary sprayed layer will crack due to a thermal stress or a mechanical shearing stress to make it impossible to prevent the sprayed layer from peeling.
In Japanese Patent KOKAI No. 87859/1986, there is proposed a process for improving the adherency between the primary sprayed layer and the parent aluminum alloy. According to this process, a base of Al or its alloy is sprayed with a primary layer of an alloy of Ni--Cr or Ni--Al, and this primary layer is formed thereon with a ceramic-sprayed layer. At the spraying step of the primary layer, the base is heated at 130.degree. to 250.degree. C. As a result of this heating, the primary layer is sprayed to extend into the base being expanded so that the adherency of the primary layer to the base is improved.
On the other hand, generally speaking, the adherency between a parent material and a sprayed layer of the alloy of Ni--Cr is improved by another process, as disclosed in Japanese Patent KOKAI No. 2872/1982. According to this process, a heat treatment is accomplished after the spray of the Ni--Cr alloy to establish a mutual diffusion between the parent material and the sprayed layer to enhance the adherency.
According to still another process, on the other hand, the parent aluminum alloy has its surface roughened in advance by a shot-blasting or grooving step, the roughened surface is then sprayed with a primary layer and further with a ceramic material. This process is expected to enhance the adherency of the primary sprayed layer by the mechanical anchoring effect of the material for the primary spray, which effect is attained by the roughness of the parent surface.
As a matter of fact, however, even the process of preheating the parent material at 130.degree. to 250.degree. C. at the step of spraying the bond layer (as has been disclosed in Japanese Patent KOKAI No. 87859/1986) has been found to result in difficulty in achieving sufficient inherence of the bond layer and the parent material and has failed to avoid the problem a peeling of the sprayed layer due to thermal stress or the mechanical shearing stress.
As is disclosed in Japanese Patent KOKAI No. 2872/1982, on the other hand, the heat treatment involved in the process for the mutual diffusion after the spray of the Ni-based alloy has to be continued at a considerably high temperature for a period as long as several tens to hundreds hours, because the diffusion is in a solid phase, so that the diffusion may be sufficient for enhancing the adherency. This long heat treatment drastically deteriorates the workability and productivity. Moreover, the solid-state diffusion of the process proposed has also found it practically difficult to complete the diffusion to achieve sufficient adherency. This difficulty in turn limits the effect of improving the adherency.
On the other hand, the process itself for improving the adherency of the primary sprayed layer by the mechanical anchoring effect which is obtained by roughening the surface of the parent material is not sufficient to achieve the required improvement and thus it also results in difficulty in preventing the sprayed layer from peeling.